1. Field of the Invention
The invention relates to communications and, more particularly, to a communication system between a pair of communication devices. The invention also relates to a communication system between a pair of programmable logic controllers.
2. Background Information
It is known to transfer a collection of information from a master device to a slave device, and/or to transfer another collection of information from the slave device to the master device.
Examples of such master and slave devices include programmable logic controllers (PLCs) which are well-known in the art. PLCs are utilized to solve ladder logic which is represented by a power rail, a plurality of inputs or functional devices or contacts, a plurality of outputs or results devices or coils, and a neutral rail. The ladder logic includes a plurality of individual ladder diagrams, segments or rungs.
In addition to contacts and coils, data values in PLCs are often organized in a series of holding registers (e.g., 16-bit words). In tun, various "special functions" are employed in order to provide mathematical or logical manipulation of the holding registers, or to send and/or receive some of the holding registers via serial communication (e.g., RS-232, communication network). For example, a block move special function may be employed to send or receive a collection of holding registers to or from another device (e.g., another PLC, another controller, an input device, an output device).
A master PLC, for example, may transfer a collection of data, such as controls, to a slave PLC, and the slave PLC may transfer a collection of data, such as indications or display information, to the master PLC. One disadvantage of these forms of communication is that, typically, most of the information being sent is "static" rather than "dynamic". In other words, between successive communications, only the dynamic data has changed. Thus, a significant portion of the bandwidth of the communication link between the master and slave devices may typically be wasted.
It is also known to transfer data between a first device and one or more second devices over a communication network within a specific time period. See, for example, U.S. Pat. Nos. 5,142,623; and 5,864,680.
In broadcast multi-cast communication networks, the messages are not addressed to any specific station. Each station receives all of the data transmitted and traditionally takes what it needs. This also requires processing of the message by the central processing unit of the receiving station. Typically, to reduce the burden on the system, data is only broadcast by exception (i.e., only when the value of a piece of data changes is a corresponding message sent). Thus, for instance, in a distributed process control system, where various functions are being performed by a number of stations on a network, data is only broadcast when the value of a variable changes. Other deterministic systems, such as automatic teller machines linked to a central unit by a network, also only send data when there is a particular transaction to process. However, such systems can become overburdened if there is a large disturbance in the process being controlled, or if a large number of transactions occur simultaneously. See, for example, U.S. Pat. No. 5,864,680.
In known transfers of collections of data between corresponding PLCs, it is believed that all of such data is transferred between the PLCs regardless whether any or all of that data has changed. For example, a large number of holding registers may be periodically sent even though only one holding register or bit, if any, has changed since the previous communication.
Although the prior art shows various types of communication systems, there is room for improvement.